The fat content of young, growing rats can be kept extremely low by regularly performed endurance exercise. The reduction in fat appears to be due to a decrease in both the size and number of fat cells found in the body. The finding that exercise is effective in depressing cell division in adipose tissue raised the possibility that exercise can curb the potential for the development of obesity of early onset which manifests itself as a marked increase in fat cell number with little change in cell size. Our plan is to elucidate the role of exercise in this area. Studies have also been designed to obtain leads regarding the basic mechanisms by which exercise modifies adipose tissue cellularity. This work will lead us to study the interrelationships between exercise and nutrition with respect to weight gain, adipose tissue cellularity, and body composition.